A boom and arm type work implement provided with a bucket has a boom mounted thereon so that it may be swung up and down by a boom actuating cylinder, an arm connected to the boom so that it may be swung up and down by an arm actuating cylinder, and a bucket connected to the arm so that it may be swung up and down by a bucket actuating cylinder, and is arranged such that the boom, the arm and the bucket are swung up and down to conduct earth excavation work.
The hydraulic circuit for operating this boom and arm type work implement is arranged such that the fluid under pressure discharged by a hydraulic pump is supplied by a boom operating valve into the boom actuating cylinder, the fluid under pressure is supplied by an arm operating valve into the arm actuating cylinder, and the fluid under pressure is also supplied by a bucket operating valve into the bucket actuating cylinder.
The hydraulic circuit for supplying the fluid under pressure discharged by a hydraulic pump by an operating valve into a work implement lifting side chamber and a work implement lowering side chamber of each of the work implement actuating cylinders so as to extend and retract the piston rod in each of the cylinders is well known.
As the operating valves for use with such a hydraulic circuit, a closed-center type operating valve is heretofore known. This closed-center type operating valve is suitable for use in case a plurality of operating valves are operated simultaneously to supply the fluid under pressure discharged by a single hydraulic pump into a plurality of hydraulic cylinders, since when the operating valve is located at its neutral position the pump port thereof is shut off.
The closed-center operating valve has a neutral position where a pump port, a tank port, a first port, and a second port are shut off, a first actuating position where the pump port is communicated with the first port, and the tank port is communicated with the second port, and a second actuating position where the pump port is communicated with the second port, and the tank port is communicated with the first port. This operating valve is arranged such that it is changed over to each of the above-mentioned positions when a spool slidably inserted in the valve body is moved; that is, when the spool is moved from its neutral position towards its first actuating position the tank port is communicated with the second port to thereby open the metering-out side, and when the spool is further moved in the same direction the pump port is communicated with the first port to thereby open the metering-in side, and the area of opening of each port is increased in proportion to the stroke of the spool. This effect can be seen with reference to FIG. 1.
This is applicable to the case where the spool is changed over to the second actuating position.
Further, there are cases where the boom of a boom-and-arm type work implement is lowered by its own weight so as to bring the bucket into contact with the ground to conduct earth compacting operation. In such cases, the boom operating valve is operated from its neutral position to a position where the metering-out side is opened and the metering-in side is opened slightly so as to lower the boom by its own weight.
However, the stroke of the spool which occurs until the metering-in side is opened after the metering-out side is opened is very short, as shown in FIG. 1, and therefore the spool is sometimes moved to a position where the metering-in side is widely opened and the pressure within the boom lowering side chamber is raised with the result that the boom is lowered forcibly by the action of the boom actuating cylinder. As a result, the bucket is vigorously pushed against the ground thus raising the vehicle body, which makes it difficult to conduct earth compacting operation using the bucket.
In brief, even if the stroke of the spool until the metering-in side is opened after the metering-out side is opened is increased, the piston rod in the boom actuating cylinder cannot be retracted until the metering-in side is opened, and therefore it is required to move the spool until the meterin-in side is opened slightly.
Further, in cases where operations other than the above-mentioned earth compacting operation are conducted, quick operation of the work implement is required to conduct the operations quickly.
In order to increase the operating speed of the boom actuating cylinders in the above-mentioned hydraulic circuit, the arrangement is made such that the fluid under pressure returning from the boom lifting side chamber is supplied partially into the boom lowering side chamber so as to quickly extend and retract the piston rod in the boom actuating cylinder.
For example, the operating valve has a fluid passage formed in the spool and a check valve so that when fluid under pressure is supplied into the boom lowering side chamber of the boom actuating cylinder a part of the pressurized fluid returning from the boom lifting side chamber may be supplied through the fluid passage and the check valve into the boom lowering side chamber, or alternatively a regenerative valve is provided in a connection circuit between the operating valve and the boom actuating cylinder so that the fluid under pressure returning from the boom lifting side chamber can be supplied directly into the boom lowering side chamber without through the operating valve.
In the case of the former arrangement, since the fluid passage formed in the spool is subjected to a constraint by the diameter of the spool, the sectional area of the fluid passage is limited, thus increasing the resistance to flow of fluid under pressure, which increases the pressure loss.
In the case of the latter construction, since the regenerative valve is installed separately from the operating valve, piping arrangement becomes complicated.